Java HashMap get method null pointer exception

我的代码类似于以下内容：

public class A {
        private HashMap<Character, Boolean> myMap;

        public A() {
            myMap = new HashMap<Character, Boolean>();
            String mychars = "asdfzxcvqwer";
            for (char c : mychars.toCharArray())
                myMap.put(c, true);
        }

        public void doo(String input) {
            StringBuilder output = new StringBuilder();
            for (char c : input.toCharArray()) {
                if (myMap.get(c))
                    output.append(c);
            }
        }
        //...
        //...
    }

为什么我在行中出现空指针异常 （NPE）？if (myMap.get(c))